Ask an Expert: Methane Digesters in Livestock Operations

Livestock manure from concentrated livestock operations can be a source of energy production that not only provides an alternative energy source for on-farm use, but mitigates the negative consequences of odor from livestock operations.

Biogas generated from manure can be used directly in a gas-fired combustion engine or a microturbine to create electricity. Additional energy in the form of waste heat from turbine operations can be used to provide heat or hot water for on-farm use, as well as maintain the temperature of a digester during a cold winter.

Methane (aneorobic) digestion occurs when organic material decomposes biologically in the absence of oxygen. This process releases biogas while converting an unstable, pathogen-rich, nutrient-rich organic substrate like manure into a more stable and nutrient-rich material with a reduced pathogen load.

Biogas is composed of approximately 65 percent methane with the remaining content being mostly carbon dioxide and other trace gases. The left over, more stable substrate can be a good source of fertilizer, or in some cases, further composted and reused as a bedding material.

An anaerobic digester is the unit of operation used to produce methane from manure.

In an anaerobic digester, the organic substrate is first liquefied by bacteria.
This is followed by a two-step process involving acid production by acid-forming bacteria (acidogenesis) and methane production from the acids with methane-forming bacteria (methanogenesis).

In most cases, after digestion the effluent can be relatively easily separated into solid and liquid fractions. In the case of dairy cows, the solid fraction may be used as recycled bedding, and the rest of the digested material may be land-applied at the agronomic rate to meet the soil and crop needs.